# 7.3. Comparison operators (binary)

integer comparison

-eq

is equal to

if [ "\$a" -eq "\$b" ]

-ne

is not equal to

if [ "\$a" -ne "\$b" ]

-gt

is greater than

if ["\$a" -gt "\$b" ]

-ge

is greater than or equal to

if [ "\$a" -ge "\$b" ]

-lt

is less than

if [ "\$a" -lt "\$b" ]

-le

is less than or equal to

if [ "\$a" -le "\$b" ]

<

is less than (within double parentheses)

(("\$a" < "\$b"))

<=

is less than or equal to (within double parentheses)

(("\$a" <= "\$b"))

>

is greater than (within double parentheses)

(("\$a" > "\$b"))

>=

is greater than or equal to (within double parentheses)

(("\$a" >= "\$b"))

string comparison

=

is equal to

if [ "\$a" = "\$b" ]

==

is equal to

if [ "\$a" == "\$b" ]

This is a synonym for =.

 ``` 1 [[ \$a == z* ]] # true if \$a starts with an "z" (pattern matching) 2 [[ \$a == "z*" ]] # true if \$a is equal to z* 3  4 [ \$a == z* ] # file globbing and word splitting take place 5 [ "\$a" == "z*" ] # true if \$a is equal to z* 6  7 # Thanks, S.C.```

!=

is not equal to

if [ "\$a" != "\$b" ]

This operator uses pattern matching within a [[ ... ]] construct.

<

is less than, in ASCII alphabetical order

if [[ "\$a" < "\$b" ]]

if [ "\$a" \< "\$b" ]

Note that the "<" needs to be escaped within a [ ] construct.

>

is greater than, in ASCII alphabetical order

if [[ "\$a" > "\$b" ]]

if [ "\$a" \> "\$b" ]

Note that the ">" needs to be escaped within a [ ] construct.

See Example 26-6 for an application of this comparison operator.

-z

string is "null", that is, has zero length

-n

string is not "null".

 The -n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [1]

Example 7-5. arithmetic and string comparisons

 ``` 1 #!/bin/bash 2  3 a=4 4 b=5 5  6 # Here "a" and "b" can be treated either as integers or strings. 7 # There is some blurring between the arithmetic and string comparisons, 8 #+ since Bash variables are not strongly typed. 9  10 # Bash permits integer operations and comparisons on variables 11 #+ whose value consists of all-integer characters. 12 # Caution advised. 13  14 echo 15  16 if [ "\$a" -ne "\$b" ] 17 then 18  echo "\$a is not equal to \$b" 19  echo "(arithmetic comparison)" 20 fi 21  22 echo 23  24 if [ "\$a" != "\$b" ] 25 then 26  echo "\$a is not equal to \$b." 27  echo "(string comparison)" 28  # "4" != "5" 29  # ASCII 52 != ASCII 53 30 fi 31  32 # In this particular instance, both "-ne" and "!=" work. 33  34 echo 35  36 exit 0```

Example 7-6. testing whether a string is null

 ``` 1 #!/bin/bash 2 # str-test.sh: Testing null strings and unquoted strings, 3 # but not strings and sealing wax, not to mention cabbages and kings... 4  5 # Using if [ ... ] 6  7  8 # If a string has not been initialized, it has no defined value. 9 # This state is called "null" (not the same as zero). 10  11 if [ -n \$string1 ] # \$string1 has not been declared or initialized. 12 then 13  echo "String \"string1\" is not null." 14 else 15  echo "String \"string1\" is null." 16 fi 17 # Wrong result. 18 # Shows \$string1 as not null, although it was not initialized. 19  20  21 echo 22  23  24 # Lets try it again. 25  26 if [ -n "\$string1" ] # This time, \$string1 is quoted. 27 then 28  echo "String \"string1\" is not null." 29 else 30  echo "String \"string1\" is null." 31 fi # Quote strings within test brackets! 32  33  34 echo 35  36  37 if [ \$string1 ] # This time, \$string1 stands naked. 38 then 39  echo "String \"string1\" is not null." 40 else 41  echo "String \"string1\" is null." 42 fi 43 # This works fine. 44 # The [ ] test operator alone detects whether the string is null. 45 # However it is good practice to quote it ("\$string1"). 46 # 47 # As Stephane Chazelas points out, 48 # if [ \$string 1 ] has one argument, "]" 49 # if [ "\$string 1" ] has two arguments, the empty "\$string1" and "]" 50  51  52  53 echo 54  55  56  57 string1=initialized 58  59 if [ \$string1 ] # Again, \$string1 stands naked. 60 then 61  echo "String \"string1\" is not null." 62 else 63  echo "String \"string1\" is null." 64 fi 65 # Again, gives correct result. 66 # Still, it is better to quote it ("\$string1"), because... 67  68  69 string1="a = b" 70  71 if [ \$string1 ] # Again, \$string1 stands naked. 72 then 73  echo "String \"string1\" is not null." 74 else 75  echo "String \"string1\" is null." 76 fi 77 # Not quoting "\$string1" now gives wrong result! 78  79 exit 0 80 # Also, thank you, Florian Wisser, for the "heads-up".```

Example 7-7. zmost

 ``` 1 #!/bin/bash 2  3 #View gzipped files with 'most' 4  5 NOARGS=65 6 NOTFOUND=66 7 NOTGZIP=67 8  9 if [ \$# -eq 0 ] # same effect as: if [ -z "\$1" ] 10 # \$1 can exist, but be empty: zmost "" arg2 arg3 11 then 12  echo "Usage: `basename \$0` filename" >&2 13  # Error message to stderr. 14  exit \$NOARGS 15  # Returns 65 as exit status of script (error code). 16 fi 17  18 filename=\$1 19  20 if [ ! -f "\$filename" ] # Quoting \$filename allows for possible spaces. 21 then 22  echo "File \$filename not found!" >&2 23  # Error message to stderr. 24  exit \$NOTFOUND 25 fi 26  27 if [ \${filename##*.} != "gz" ] 28 # Using bracket in variable substitution. 29 then 30  echo "File \$1 is not a gzipped file!" 31  exit \$NOTGZIP 32 fi 33  34 zcat \$1 | most 35  36 # Uses the file viewer 'most' (similar to 'less'). 37 # Later versions of 'most' have file decompression capabilities. 38 # May substitute 'more' or 'less', if desired. 39  40  41 exit \$? # Script returns exit status of pipe. 42 # Actually "exit \$?" unnecessary, as the script will, in any case, 43 # return the exit status of the last command executed.```

compound comparison

-a

logical and

exp1 -a exp2 returns true if both exp1 and exp2 are true.

-o

logical or

exp1 -o exp2 returns true if either exp1 or exp2 are true.

These are similar to the Bash comparison operators && and ||, used within double brackets.
 ` 1 [[ condition1 && condition2 ]]`
The -o and -a operators work with the test command or occur within single test brackets.
 ` 1 if [ "\$exp1" -a "\$exp2" ]`

Refer to Example 8-3 and Example 26-11 to see compound comparison operators in action.

### Notes

 [1] As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n "\$string" -o "\$a" = "\$b" ] may cause an error with some versions of Bash if \$string is empty. The safe way is to append an extra character to possibly empty variables, [ "x\$string" != x -o "x\$a" = "x\$b" ] (the "x's" cancel out).